<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 300px;
            background-color: rgba(0,0,0,0.3);
            padding: 10px;
            position: relative;/*做参照物*/
        }
        input{
            border: none;padding: 10px 20px;width: 300px;
            box-sizing: border-box;
        }
        img{
            position: absolute;
            top: 14px;
            right: 30px;
        }
        p{
            font-size: 12px;
            color: red;
            margin: 0;
        }
    </style>
</head>
<body>
<!--
div宽300    背景黑色透明度0.3
内边距10
文本框去掉边框 border:none
内边距上下10左右20
字红色12个像素
-->
<div>
  <input type="text" placeholder="请输入用户名">
  <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
  <p>用户名不能为空!</p>
</div>
</body>
</html>